Geological Time

The time scale at left is both a reference and a key to the
display cases at the museum ­ click on a geologic period
to go to that case. Note that in the United States it
is common to break the Carboniferous into two periods, the Pennsylvanian
and the Mississippian, as is done in our museum. The Museum
thanks Dr. Andrew MacRae for the use of the time scale image and
the short essay below.

Few
discussions in geology can occur without reference to geologic
time. Geologic time is usually discussed in two forms:

Relative time - named subdivisions of the Earth's
geology in a specific order (for example, the "Cambrian
Period", followed by the "Ordovician Period",
and "Silurian Period"). Most of these subdivisions
are recognized globally on the basis of their relative position
in the Earth's stratigraphy and their fossil content.

Absolute time - numerical ages, often expressed in
"millions of years before present". These are
most commonly obtained by radiometric dating methods performed
on appropriate rock types.

The two types of geologic time are analogous to the difference
between "lunchtime" ("relative time") and
the numerical time on a clock, like 12:00pm to 1:00pm ("absolute
time"). "Lunchtime" occurs after "morning"
and before "suppertime", but its position in time and
its duration can also be measured in hours and minutes, just like
the Jurassic Period occurs after the Triassic Period, and before
the Cretaceous Period, and spans the time from about 205 million
years ago to about 142 million years ago.

Relative time is the physical subdivision of the rocks found
in the Earth's geology and the time and order of events they represent.
Absolute time is the measurement taken from the same rocks to
determine the amount of time that has expired. Absolute
time measurements can therefore be used to calibrate the
relative time scale, producing an integrated geologic or "geochronologic"
time scale that combines both types of data, as is depicted here.

This geologic time scale is based upon data from Harland
et al., (1990) and Gradstein
and Ogg, (1996). The time scale is depicted in its traditional
form with oldest at the bottom, and youngest at the top ­
the present day is at the zero mark. The scale is broken
in the Precambrian because this period is extremely long in duration
(it extends from 545 million years ago to over 4.5 billion years
ago). An image with a more
complete timescale is also available, as is more
information and references about geologic time scales.